This invention relates generally to a power transfer mechanism adapted to be incorporated in a multiple path drive system, for example a four-wheel drive vehicle. More particularly, it relates to a torque transfer case adapted to receive torque from a prime mover and to provide torque for one drive axle where two-wheel drive is adequate, and to provide torque for both drive axles automatically where four-wheel drive is required. The torque transfer case incorporates a double-acting overrunning clutch responsive to relative rotation between the drive axles for automatically engaging where four-wheel drive is required.
In recent years there have been many improvements in automotive drive trains, including improvements relating to the transfer of torque from a prime mover to drive axles. Where four-wheel drive systems are used, transfer cases have been developed which generally provide torque transfer to one output to drive an axle and to another output for driving another axle. Some such transfer cases generally have included an overrunning mechanism which automatically engages and disengages the four-wheel drive function by biasing the cage of the mechanism to ground. It has been found that an excessive overrunning condition may cause undesirable lock-up of the overrunning mechanism, thus negating its automatic function. This problem has been partially solved by the use of a speed-responsive blocking structure as disclosed in U.S. Pat. application Ser. No. 615,001 filed Sept. 19, 1975. However, it is now known that an excessive overrunning condition may obtain at speeds below the threshold speed of such a structure. Thus, there remains a need to provide an improved torque transfer case which engages and disengages automatically, but which prevents undesirable engagement even at low speeds, while providing for smooth operation, low wear and equalization of load on the transfer mechanism.
Accordingly, it is an object of this invention to meet the continuing need and desire in the art for improvements in a torque transfer mechanism which normally provides torque transfer to one output shaft for establishing a two-wheel drive condition, and which automatically engages through an overrunning device to provide torque transfer to two output shafts for establishing a four-wheel drive condition. The overrunning device is biased toward engagement, but normally remains disengaged so long as an overrunning condition exists. The mechanism includes a unique direction-responsive blocking construction for preventing undesirable engagement should an excessive overrunning condition be encountered.